chore: remove dead code, tighten types, fix lint warnings

[ZacharyZcR]

Summary

Systematic cleanup across 89 files — no functional changes, net -178 lines.

Dead code removal

• Remove unused imports (guacamole-server, opkssh-auth, auth-manager, DesktopApp, FullScreenAppWrapper, etc.)
• Remove unused variables (skipped in PollingManager, isAdmin in FullScreenAppWrapper, cores reassignment in cpu-collector)
• Remove dead handlePasswordReset function and related state in UserEditDialog — the function and showPasswordReset were defined but never referenced in JSX (the button was already gone)
• Remove unused props (allowPasswordLogin, currentUser in DatabaseSecurityTab, onOpenCommandPalette in TopNavbar)

Type tightening (as any → 0)

• Replace all as any casts in changed files with proper types (72 → 0)
• Add ParsedPrivateKey, PublicKeyAuthInfo, OPKSSHProtocol, OPKSSHClient interfaces for opkssh-cert-auth
• Add HttpLikeError interface for db-health-monitor
• Add TerminalRefHandle interface and type TabContextTab.terminalRef as RefObject<TerminalRefHandle>
• Type window.opera access, toast options, event listeners, connection log details
• useIsMobile: eliminate initial undefined state with lazy initializer
• SidebarMenuSkeleton: useMemo → useState for random width (computed once, not reactive)

Error chain preservation

• Add { cause: error } to 15+ throw new Error(...) calls across backend utilities (system-crypto, database-file-encryption, database-migration, auto-ssl-setup, etc.)

Lint cleanup

• catch (e) → catch where error variable is unused
• let x = "" → let x: string where initial value is always overwritten
• void _next instead of eslint-disable comment for Express error handler
• Record<string, any> → Record<string, unknown> in type definitions

Metrics

Metric	Before	After
type-check	✅	✅
ESLint errors	0	0
ESLint warnings	222	187
@typescript-eslint/no-explicit-any	72 (in changed files)	0

Test plan

• App loads and functions normally (no behavioral changes)
• TypeScript compilation passes
• ESLint passes with 0 errors