fix(frontend+api): wire R/ggplot2 into code viewer, libraries page, languages count

[MarkusNeusinger]

Three R/ggplot2 rollout gaps surfaced after PR #6951 merged. All show wrong data despite the DB row being correct — they were always going to need a follow-up after the first non-Python impl landed.

1. Code viewer empty on /scatter-basic/r/ggplot2

`ImplRepository.get_code()` has signature `get_code(spec_id, library_id, language_id="python")`. The `/specs/{spec_id}/{library}/code` endpoint never accepted a language and never passed one through — so R rows (`language_id="r"`) failed the WHERE clause, the endpoint returned 404, and the frontend showed a blank code panel.

```bash
$ curl -s -o /dev/null -w "%{http_code}\n" https://anyplot.ai/api/specs/scatter-basic/matplotlib/code
200
$ curl -s -o /dev/null -w "%{http_code}\n" https://anyplot.ai/api/specs/scatter-basic/ggplot2/code
404
```

Fix:

• API: `get_impl_code` accepts `?language=` (default `"python"` for backwards compat). `_build_impl_code` and the Redis cache key both include language so Python and R impls under the same library_id can't collide.
• Frontend: `useCodeFetch` accepts an optional `language` arg, includes it in the in-memory cache key, and only appends `?language=` to the URL when it diverges from python — Python URLs and existing tests stay unchanged.
• SpecPage: passes `urlLanguage` (already destructured from `useParams`) to `fetchCode`/`getCode`, and `impl.language` to the copy-to-clipboard path.

2. Landing strip shows "languages: 1"

`NumbersStrip.tsx` line 16 was `{ value: '1', label: 'languages' }` — literally hardcoded, never wired to any stat.

Fix:

• API: add `languages: int = 0` to `StatsResponse`. `_refresh_stats` and the cached `_fetch` factory derive it from `{lib.language_id for lib in libraries}` (with a `len(LANGUAGES_METADATA)` fallback when libraries are unavailable, mirroring how libraries count already worked).
• Frontend: bind to `stats.languages` with an em-dash fallback while loading.

3. ggplot2 missing from /libraries

`app/src/constants/index.ts` had:
```ts
export const LIBRARIES = ['altair', 'bokeh', 'highcharts', 'letsplot', 'matplotlib', 'plotly', 'plotnine', 'pygal', 'seaborn'];
```

LibrariesPage iterates this list to render cards, so ggplot2 was silently skipped even though `/api/libraries` already returned it.

Fix: add `'ggplot2'` to `LIBRARIES` (alpha order, between bokeh and highcharts) and `'gg'` to `LIB_ABBREV` for compact display.

Test plan

• `useCodeFetch.test.ts` — 12/12 green (10 existing + 2 new: `?language=` is appended for non-python, separate cache keys per language for same library_id)
• `tests/unit/api/test_stats.py` — green, asserts `languages == 1` for the mocked single-library case
• `tests/unit/api/test_schemas.py` — green, `languages` field present in `model_dump()`
• `tests/unit/api/test_routers.py` — 8/8 code-endpoint tests still pass (default-python path unchanged)
• After deploy: `curl https://anyplot.ai/api/specs/scatter-basic/ggplot2/code?language=r\` returns 200 with the R source
• After deploy: `/libraries` shows ggplot2 card, landing strip shows `languages: 2`

Out of scope (separate issues)

• Issue Plot title shows library but not language #6958 — language is not shown in plot image titles (`scatter-basic · ggplot2 · anyplot.ai` is ambiguous for R vs Python libs); design decision pending
• Pre-existing TS error in `CodeHighlighter.tsx:3` for the R syntax-highlighter import (`@types/react-syntax-highlighter` doesn't declare `prism/r`) — was introduced by PR feat(ggplot2): add R/ggplot2 library + multi-language pipeline #6944, not by this PR

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Follow-up to PR #6951 fixing three rollout gaps after the first non-Python (R/ggplot2) implementation merged: the code viewer 404'd for R impls, the landing strip hard-coded languages: 1, and ggplot2 was missing from /libraries.

Changes:

• API: /specs/{spec_id}/{library}/code accepts ?language= (default python), threaded through _build_impl_code and the Redis cache key; StatsResponse gains a languages field derived from distinct library.language_ids.
• Frontend: useCodeFetch accepts an optional language, includes it in cache key, and appends ?language= to the URL only for non-Python; SpecPage passes through urlLanguage/impl.language; NumbersStrip binds to stats.languages.
• Constants: ggplot2 added to LIBRARIES (alpha order) and LIB_ABBREV (gg).

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
api/routers/specs.py	Adds language query param to code endpoint and includes it in cache key + repo lookup
api/routers/stats.py	Computes distinct-language count from libraries, with LANGUAGES_METADATA fallback
api/schemas.py	Adds languages: int = 0 to StatsResponse
app/src/hooks/useCodeFetch.ts	Optional language param; cache keyed by specId:language:library; query param only for non-python
app/src/hooks/useCodeFetch.test.ts	New tests for ?language=r URL and per-language cache separation
app/src/pages/SpecPage.tsx	Passes urlLanguage to fetchCode/getCode, and impl.language to copy-to-clipboard
app/src/components/NumbersStrip.tsx	Binds languages value to stats.languages with em-dash fallback
app/src/constants/index.ts	Adds ggplot2 to LIBRARIES and gg to LIB_ABBREV
tests/unit/api/test_stats.py	Asserts languages == 1 for single mocked library
tests/unit/api/test_schemas.py	Updates serialization assertion and adds languages field test

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
api/routers/specs.py	85.71%	1 Missing ⚠️
app/src/components/NumbersStrip.tsx	0.00%	1 Missing ⚠️
app/src/pages/SpecPage.tsx	66.66%	1 Missing ⚠️

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