kickstart ui cleanup: shared dropdown, collapsible checks, base cost estimate, region prefs

[David Gamero (davidgamero)]

kickstart ui cleanup pass — dropdowns + preflight checks. (region-pref + cost-estimate changes split out into #2327.)

dropdowns

• new shared SearchableDropdown<T> component (fuzzy search + full keyboard/a11y: arrow nav w/ wraparound, home/end, enter/escape/tab, scrollIntoView, drop-up, aria) — best-of-both of the old TextWithDropdown and CustomDropdown
• generic items via getValue / optional toLabel / optional sortKey (sortKey sets default order + tiebreaks equal fuzzy scores)
• migrated all 7 kickstart dropdowns to it (6 in KickstartCluster, 1 github-repo in guided setup)
• old components left alone for the ~12 non-kickstart usages (follow-up: migrate the rest + delete them so this is a net line reduction)
• github repo dropdown now suffixes private repos with (private); label composition moved out of jsx into toLabel

preflight checks density

• wrapped the preflight check list in a collapsible group — shows a one-line pre-flight checks — N of M passed summary
• auto-collapses once everything passes, auto-expands while running or on any fail/warn
• succeeded stages now render just title + detail (dropped the leftover clock/elapsed/action rows that made each check ~5 lines)
• de-duped the providers check (was showing during the region scan AND in preflight)
• extracted a shared DisclosureButton so the three collapse toggles (full-error, capacity-checks, stage-group) share one impl + css instead of three copies
• small spacing consistency fixes (checkDetail double-margin, permission-warning gap)

notes

• extension + webview typecheck, eslint, and npm run build all clean
• not yet runtime-tested in vscode