feat(web): quieten the sidebar session rows

[elkaix]

Related Issue

No issue. The problem is described below.

Problem

The sidebar session rows do the opposite of the two idioms the rest of the UI now follows. They hover with a solid var(--panel2) fill and mark the selected row with a solid var(--soft) fill, at an 8px radius with a pinned text size.

What changed

• Hover moves to the --hover token — the translucent wash style.css already describes for exactly this purpose.
• The selected row becomes the same faint color-mix wash the shared MenuRow primitive uses, so the sidebar and the menus agree.
• The radius moves to --r-md, and the row and title sizes derive from --ui-font-size so the font-size setting still scales them. Section labels drop to the muted tone.

Behaviour is unchanged.

Three things worth calling out:

• style.css carried its own per-theme overrides for these rows, with their own solid colours. Changing only the scoped block would have left the modern and pythinker themes untouched — the change would have looked correct in the default theme and been invisible in the other two. Those overrides are updated as well, and a test now pins them.
• The row height is a minimum, not a fixed height. A fixed height with the existing padding leaves a 13px content box, which clips the 18px tag pill and the archive-confirm strip. jsdom performs no layout, so no test could have caught this — the guard now asserts the property is min-height and fails if it becomes a fixed height.
• The colour-literal guard no longer diffs against git show HEAD. That baseline compares the file to itself once the change lands, so it could never fail again, and it required git at test time. It is now an explicit allowlist of the one pre-existing literal.

Deliberately not included: the sidebar resizer and the chat header. App.vue already has a persisted drag resizer via ResizeHandle/useResizable (270px default, clamped 170–420) — rewriting it to different numbers is churn. The header's fixed 48px is coupled to the desktop title-bar reserve asserted in windows-titlebar.test.ts, so restyling it belongs in its own change.

Verified locally: 346 web tests, typecheck and lint all pass, and the suite passes again after the pre-commit autofix. Both new guards were mutation-checked.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

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commit: 93e31d7

[elkaix]

Closing: merged locally into main; a new PR will follow.