fix(web): use message-square icon for settled icon-less project threads in sidebar v2

[UtkarshUsername]

What Changed

• ProjectFavicon gained an optional fallbackIcon prop (defaults to FolderIcon)
• Settled threads (slim rows) in sidebar v2 now pass MessageSquareIcon as the fallback
• Card variant (active threads) unchanged. FolderIcon beside the project name stays correct

Why

Settled threads show the project favicon beside the thread title. When a project has no favicon, FolderIcon was shown, which is misleading next to a thread name (it implies the thread is a folder). MessageSquareIcon was already used elsewhere in the codebase for thread-related commands, so is a natural fit and visually matches the small icon size.

UI Changes

Before:

After:

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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Note

Low Risk
Small presentational change with a backward-compatible optional prop; no auth, data, or API impact.

Overview
ProjectFavicon now accepts an optional fallbackIcon prop (still defaults to FolderIcon) and uses it whenever there is no favicon URL, including while an image is loading or after load errors.

In sidebar v2, slim (settled) thread rows pass MessageSquareIcon as that fallback so icon-less projects show a chat glyph next to the thread title instead of a folder. Card rows and other ProjectFavicon call sites are unchanged and keep the folder fallback beside project names.

^{Reviewed by Cursor Bugbot for commit 154821a. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Use message-square icon for icon-less project threads in sidebar v2

Adds a fallbackIcon prop to ProjectFavicon, ProjectFaviconFallback, and ProjectFaviconImage in ProjectFavicon.tsx so callers can supply a custom icon instead of the default FolderIcon. In SidebarV2.tsx, thread rows now pass MessageSquareIcon as the fallback, so a chat bubble appears when a thread has no favicon rather than a folder.

^{Macroscope summarized 154821a.}

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[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

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[macroscopeapp]

Approvability

Verdict: Approved

This PR adds an optional fallback icon prop to the ProjectFavicon component and uses it to show a message-square icon instead of a folder icon in the sidebar. The change is purely cosmetic with no runtime behavior impact.

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