fix(web): share compact CTA hover and inline showcase keycaps

[tyler-dane]

Summary

Finishes the two cleanups PR #2770 deferred, plus the PostHog follow-up for the assist event rename.

• Compact CTAs share hover:bg-accent-hover. c-button stays the tall elevated default (h-11 + lift). A new c-button-compact utility is the row-height variant: focus ring, disabled state, no forced height or shadow. Call sites in the checklist, welcome pills, anonymous calendar tooltip, calendar list header, and tasks-removal notice now pair it with c-button-primary (or c-button-secondary for Log in). Changing --color-accent-hover reaches all of them. The welcome Log in pill drops hardcoded hex in favor of c-button-secondary.
• Undo/redo chips live in the sentence. ShowcaseStep.body can be the existing ShortcutTipPart[] model. Only undoRedo uses it — converting every step would duplicate the chip row already rendered from keycaps. ShortcutTipParts is the shared renderer (sidebar tips now use it too). MOD_KEY / detectPlatform() are gone from showcase.steps.ts, and redo finally renders via KEYMAP.redo.keycaps. Accessible copy expands Mod to Cmd/Ctrl the same way the chips do.
• PostHog dashboard Onboarding: no existing tile queried shortcut_showcase_assist_shown. Added Showcase assist used by step (shortcut_showcase_assist_used broken down by step) and Showcase time on step (median seconds between consecutive shortcut_showcase_step_completed events, using started/redone as the previous beat). Both stay empty until production traffic exists. Did not restore the idle/failed-attempt assist hook.

Simplicity

c-button-compact rather than stripping height from c-button: the default CTA still wants 44px and the lift/shadow; compact sidebar/checklist pills do not. Optional parts on body rather than reshaping every step.

Automated validation

• Focused web tests for showcase, keymap parity, tips, welcome modal, and compact-CTA call sites: 48 pass.
• bun test:web: 2190 pass, 0 fail.
• bun run type-check: clean.
• bun run lint: 11 pre-existing warnings, none in changed files.
• Browser (Playwright against bun run dev:web): welcome Sign up 30px, Log in 32px (bordered secondary, not hex gray), checklist Sign up 28px, anon tooltip Sign up 23px. Checklist hover background rgb(135, 203, 230) vs rest rgb(124, 198, 228) — bg-accent-hover, not brightness. Undo/redo step shows inline Ctrl/Shift/Z chips in the sentence with no duplicate keycap row; sr-only copy is Ctrl+Z / Ctrl+Shift+Z. Console errors were only backend ERR_CONNECTION_REFUSED (anonymous IndexedDB mode).

Independent review

A read-only reviewer flagged that sr-only undo/redo copy still said Mod. Fixed by expanding Mod to Cmd/Ctrl in getPartsPlainText. No other confirmed defects.

Test plan

bun test:web
bun run type-check
bun run lint

Browser: welcome Sign up / Log in hover, showcase step 9 inline undo+redo chips, checklist accent Sign up pill, anon calendar tooltip Sign up.