Phase 1 + Phase 2: Drop external Bootstrap dependency, add opt-in Modern theme

[mike-thompson-day8]

Summary

This PR is the merged outcome of the long-running feature/modern-theme-phase-2 branch — two phases of work shipping together for 2.30.0:

Phase 1 — Bootstrap as external dependency removed. Re-com no longer requires consumers to load bootstrap.css separately. The Bootstrap 3.3.5 rules re-com depends on are vendored verbatim into re-com.css (preserved byte-for-byte against the upstream Bootstrap 3.3.5 source — verified). The classic re-com look is unchanged for any consumer who doesn't change their index.html.

Phase 2 — Opt-in Modern theme. New re-com.theme.modern namespace ships a Bootstrap-5-flavoured visual theme. Activate from app code with (re-com.core/reg-theme re-com.theme.modern/theme). Typography is intentionally inherited from the classic look so apps adopting Modern incrementally don't see text re-flow.

The popover modernization that PR #372 merged separately is also in this branch's history (via merge from master) but is not part of this PR's delta.

What changes

• run/resources/public/assets/css/re-com.css (+6821 lines) — vendored Bootstrap 3.3.5 base rules (header-comment-delimited section). The original re-com rules are byte-identical to the prior version.
• src/re_com/theme/modern.cljs (+670 lines, new) — Modern theme with styles multimethod. Variables layer (BS5 palette, 4px spacing scale, BS5 radii, refined shadows, focus ring colours) plus per-component defmethod styles for: button, alert-box, progress-bar, input-text, checkbox, radio-button, horizontal/pill/bar tabs, dropdown, selection-list, modal-panel, popover (regular only), datepicker, daterange, slider, input-time, h-split, v-split, hyperlink, close-button.
• run/resources/public/index_dev.html / index_prod.html — removed the external <link href="bootstrap.css">.
• demo/re_demo/theme_picker.cljs (+33 lines, new) — title-bar dropdown for live Classic ↔ Modern switching using a wrapping theme function that derefs an atom inside the render path (reactive without remounting).
• demo/re_demo/utils.cljs — adds the theme-picker to the panel-title bar.
• demo/re_demo/core.cljs — small change to mount the picker.
• demo/re_demo/customization.cljs / theme.cljs / popovers.cljs — doc updates to reflect Phase 1 + Phase 2 reality (replaces stale "bootstrap will become optional" wording, adds Modern theme callouts, updates popover-component-hierarchy table for the renames from PR Add full :parts & :theme support to popover components #372).
• CHANGELOG.md — 2.30.0 (Unreleased) entry covering Phase 1, Phase 2, and migration notes (including the CSS load-order edge case).
• package.json / package-lock.json — small demo-side dep tweaks.
• README.md — install instructions reflect that Bootstrap is no longer required.

Modern theme component coverage

Has Modern styling: button, alert-box, progress-bar, input-text, checkbox, radio-button, horizontal-tabs, pill-tabs, bar-tabs, dropdown, selection-list, modal-panel, popover (regular), datepicker, daterange, slider, input-time, h-split, v-split, hyperlink, close-button.

Unaffected by Modern (deliberate):

• Layout components (h-box, v-box, box, gap, line, scroller, border) — pure layout, untouched.
• Typography components (title, p, label) — typography is inherited from classic to avoid text re-flow.
• popover-tooltip — keeps its classic dark-on-light look (tooltips are a distinct visual element).
• Bootstrap-variant button classes (btn-primary, btn-success, btn-info, btn-warning, btn-danger, btn-link) — preserved so colour-coded buttons still get their colour.

Migration (also in CHANGELOG)

• Consumers who only loaded Bootstrap because re-com required it can remove the Bootstrap stylesheet <link>.
• Consumers who passed raw Bootstrap class names (:class "btn-primary", :class "alert-info", etc.) — those classes still resolve because the rules are now in re-com.css.
• Consumers with custom theme functions emitting Bootstrap class names ((tu/class props "btn")) — unchanged, the classes are still styled.
• CSS load-order edge case: if your app has its own stylesheet that overrides Bootstrap selectors (e.g. customising .btn or .popover), make sure it loads after re-com.css. Previously you could rely on bootstrap.css → your-overrides.css → re-com.css, but now the Bootstrap rules live inside re-com.css, so user overrides need to come after it to win the cascade.

Test plan

• bb watch compiles clean (0 warnings) — both :demo and :browser-test builds
• CSS byte-equivalence verified: extracted the original re-com rules from the post-vendoring re-com.css and diffed against the prior re-com.css content — byte-identical (only the new "vendored Bootstrap" section header differs)
• Vendored Bootstrap content verified: extracted the vendored block and diffed against bootstrap.css 3.3.5 from the cloudflare CDN — byte-identical
• Cross-page Playwright sweep of computed styles for buttons, inputs, checkboxes, dropdowns, alerts, progress bars, titles, h-boxes, v-boxes against the prod deploy — zero diffs in classic mode
• Theme picker flips Classic ↔ Modern at runtime without remounting — verified live
• Per-component visual sanity checks under Modern (popover, alert, button, tabs, dropdown, datepicker, etc.)
• Validation pass: 0 page errors, 0 re-com validation warnings on the popover demo (which exercises modernised popover parts)

Out of scope

• Pruning the vendored Bootstrap CSS — verbatim vendoring is intentional for this release. A follow-up could remove genuinely-dead sections (Glyphicons, responsive utilities, grid system, print styles, replaced components like carousel/jumbotron/navbar) for ~30–40% size reduction, but pruning has consumer-breakage risk that should be evaluated separately.
• Porting Modern styling to v-table / simple-v-table — these don't yet have parts/theme migration.