Triage: Add durable project themes

[time-attack]

Tracking the change proposed in #401 (by @wcatech).

We're moving this from a code PR to a tracked issue under CONTRIBUTING.md: feature ideas belong in adrs/ as a short human-written note and bugs as an issue, and we implement the underlying code from our side. Capturing the original proposal here so it isn't lost.

Original PR: #401
Reported by: @wcatech

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Summary

• persist a six-palette theme preset on each project
• let project owners choose Personal, Light, or Dark appearance per project
• apply the project appearance across project details, conversations, split panes, and scoped files
• restore the user's personal appearance outside a project

Screenshot

Synthetic project data:

Checks

• root project and route tests: 12 passed
• web theme, proxy, and split-pane tests: 14 passed
• root and web typechecks
• scoped ESLint
• git diff --check

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If we ship a fix or feature from this, credit @wcatech as co-author.