feat(app): add desktop setting to toggle file changes display

[anduimagui]

Issue for this PR

Closes #9089 (desktop-specific)

Type of change

• New feature

What does this PR do?

Adds a desktop-only setting to show or hide the "Changed file +N -M" diff summary in the session timeline. This gives users a cleaner chat view when they don't want the file-change accordion taking up vertical space.

Related work:

• Feature Request: TUI option for minimal/collapsed diff display #9089 — Feature Request: TUI option for minimal/collapsed diff display. This was the original request for the TUI, assigned to thdxr, with strong community support.
• feat(tui): add minimal diff display mode #9104 — Prior implementation attempt (feat(tui): add minimal diff display mode) by LIHUA919 that added tui.diff_display config. It was closed due to staleness after 60 days without maintainer review.
• fix(desktop): collapse full-context git diffs #29928 — fix(desktop): collapse full-context git diffs (open). This is related but different scope — it fixes rendering of full-context generated patches in the Git Changes sidebar, not the timeline header.
• Feature Request: TUI option for minimal/collapsed diff display #9089 remains open as the canonical issue for this feature. This PR brings the same toggle capability to the desktop surface.

Changes:

• packages/app/src/context/settings.tsx — adds showFileChanges boolean to general settings (default true)
• packages/app/src/i18n/en.ts — adds English translation keys for the new setting
• packages/app/src/components/settings-general.tsx — adds a desktop-gated <Switch> toggle in Settings > General
• packages/app/src/pages/session/message-timeline.tsx — wraps DiffSummary timeline rows with a reactive <Show> so they hide/show instantly when the setting changes

Why it works:
The toggle uses the existing settings.general persistence layer (already used by showReasoningSummaries, showSessionProgressBar, etc.). In the timeline, <Show when={...}> creates a SolidJS reactive subscription, so the rows update in real time without waiting for a new message or re-render.

How did you verify your code works?

• bun typecheck in packages/app passes (pre-existing unrelated errors in stats-server/stats-core)
• No new tests added — follows the existing settings pattern used by other boolean toggles

Screenshots / recordings

Not included.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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