[FEATURE]: Add a Refresh file index command that forces a search-index rescan

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Summary

The file search index is built once when a project opens; files added afterwards may not be discoverable. Add an explicit Refresh file index command that forces a rescan of the current project and rebuilds the search index, with progress feedback so the user knows it is running.

Related existing issues

• #35841 — TUI file index refresh on demand. This request covers the same capability but also includes: a location-scoped server endpoint that waits for the rescan to complete (not fire-and-forget), a desktop app command with an indeterminate progress indicator, and the underlying fix so that refreshed files are actually picked up by @ mentions (see #38065 / #32747).
• #38065, #32747 — @ autocomplete does not detect newly created files. The refresh command directly addresses the manual workaround for these.

Proposed scope

• Server: add a location-scoped refresh endpoint that triggers a rescan and waits for it to finish before responding (not fire-and-forget — otherwise the user sees a success toast before the index is actually rebuilt).
• Desktop app: add a Refresh file index command to the command palette; while it runs, show an indeterminate progress indicator (e.g. bottom of the window), then hide it.
• TUI: add an fs.refresh command with a keybind (e.g. F5).

Implementation notes

• ripgrep index layer: if refresh just forks a background scan and returns immediately, the user gets a false "done"; it should wait for the scan to complete.
• fff index layer: waitForScan only waits for an already-started scan — it does not trigger a new one. A real rescan API (e.g. scanFiles) must be called, otherwise refresh is a no-op.