fix(css): revert #257 @layer wrapping — sidebar content was centered

[Astro-Han]

Symptom

The sidebar nav rows (新任务 / 每日回顾 / 技能 / 定时任务), session-list rows, and the settings button all rendered with their content centered when they should be left-aligned. It's subtle in a narrow sidebar and increasingly obvious the wider the sidebar is dragged.

Root cause

#257 (cbdf1088) wrapped styles.css component rules into @layer base / @layer components, described as "pure structural wrapping." But Tailwind v4 orders the cascade base → components → utilities, so utilities have the highest priority.

The sidebar nav rows render as <UiButton size="nav" className="maka-nav-row">: the cva button base always carries the Tailwind utilities inline-flex items-center justify-center, and .maka-nav-row (display: grid + text-align: left) only overrode them while it stayed unlayered. Once moved into @layer components, it lost to the utilities → content centered. The compensating !important cleanup was deferred to "Round H," so the override has nothing to lean on today.

Evidence (CDP toggle)

Injecting a rule into the live renderer with the same selector and specificity as .maka-nav-row, differing only in being unlayered:

State	.maka-nav-row computed display
Layered (#257 as-is)	flex (the inline-flex utility wins)
Unlayered injection	grid (.maka-nav-row wins, layout restored)
Injection removed	flex (centered again)

Same rule, one variable (layer membership) → the cause is the cascade layer, not specificity.

Change

• Revert refactor(css): wrap styles.css rules into @layer base + components (Round A, #253) #257 (removes the 5 lines of @layer wrapping), restoring the unlayered cascade the whole stylesheet's override pattern depends on. Fixes every affected element at once (nav rows, session rows, settings button).
• Adds a regression contract testrenderer-style-layer-cascade-contract.test.ts: asserts .maka-nav-row is not nested in any @layer. Red on cbdf1088, green after the revert.

Verification

• Real window: all four nav labels share a uniform 42px left offset (if centered, different label lengths would start at different offsets), display: grid, and session rows are left-aligned too.
• npm run -w @maka/desktop test: this PR adds zero new failures. The 16 failures already present in the suite are pre-existing on origin/main — stale contracts introduced by ancestors of refactor(css): wrap styles.css rules into @layer base + components (Round A, #253) #257 (e.g. BOT_BRAND moved to @maka/ui, a changed copyFile signature, a radius value) — and are out of scope here.

Known trade-off (reviewed)

A full revert also returns the global form/button resets (button/input/textarea/select { font: inherit }, the global button { transition }) to unlayered, so they again outrank Tailwind text utilities. Measured impact vs #257: ~85 controls go from font-weight: 500 back to 400, and ~48 controls go from 13.125px (text-sm) back to the inherited 15px. This is not a new regression — it is the exact control typography the app shipped for months before #257; #257 incidentally flipped it one day ago alongside the sidebar break. Restoring it wholesale is the intended behavior of this revert. Whether controls should let text-sm / font-medium win is a deliberate design decision deferred to the sidebar-redesign / CSS-layering-governance follow-up, not this hotfix.

Follow-up (not in this PR)

• If Round A (styles.css 优化 #253) re-lands, component-override rules must sit in a layer declared after utilities (or stay unlayered) — not in @layer components.
• Sidebar redesign + governance of the shadcn-primitive vs bespoke-CSS styling authority is tracked separately.