Add setting to show timestamps on every message

[cwyptt]

Description

By default, Sable only shows a timestamp on the first message in a group of consecutive messages from the same sender. This adds a showAllTimestamps setting (default: off) that renders a timestamp on every message, even grouped/collapsed ones. The toggle is in Settings → General → Messages, below "Message Spacing".

When enabled, collapsed messages render a right-aligned timestamp in place of the empty header, without the username, avatar, or pronouns that appear on ungrouped messages. The layout direction respects the same Row/RowReverse logic as the full header (compact and right-aligned bubble layouts).

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

The implementation was AI-assisted: adding the showAllTimestamps boolean to the Settings interface and defaults, reading it via useSetting in Message.tsx and conditionally rendering a timestamp-only header for collapsed messages, and adding the SettingToggle in General.tsx. I reviewed each change, verified the collapsed header layout matches the existing direction logic, and ran typecheck, lint, format, and tests in a containerized environment.

[Th-Underscore]

A very similar feature is included in my local branch I'll PR in the near future once I manually clean and review everything; the timestamp appears on hover but has no opt-in config: dev...Th-Underscore:Sable:feat/message-edit-timestamps

Let me know if this PR #1942 gets merged soon so I can remove the timestamps bit from my feature set.

[7w1]

oops didn't check ci. The settings link file needs to be updated https://github.com/SableClient/Sable/blob/dev/src/app/features/settings/settingsLink.ts

[cwyptt]

ahhh, thank you @7w1; should be g2g now