feat: add Activity Indicators setting to control session progress bar animation

[jasonmj]

Issue for this PR

Closes #

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The animated progress bar that runs across the top of the session header while a session is active is distracting for some users. There was no way to disable it.

This adds a boolean setting showSessionProgress (default true) to the general settings store, wires it to a Switch toggle in Settings → General → Appearance labelled "Activity Indicators", and gates the session-progress bar render on that value in message-timeline.tsx. The spinner next to the session title is not affected. i18n keys are added to all 17 locale files with English strings as placeholders for non-English locales.

How did you verify your code works?

• Toggled the setting off and confirmed the animated bar no longer appears during a session run
• Confirmed the spinner next to the session title still appears when the setting is off
• Toggled the setting back on and confirmed the bar reappears
• Ran the i18n parity test: bun test --cwd packages/app ./src/i18n/parity.test.ts — 1 pass, 0 fail

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Checklist

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

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