Settings: add personal preferences page and wire defaults

[KucharczykL]

Summary

• add the authenticated personal settings page and navbar entry using the shared responsive settings kit
• wire per-user currency and device defaults through every purchase/session path from issue Settings epic — Stage 4: /settings page + wire easy prefs (owns shared module + navbar) #385
• validate landing-page URL names and support Sessions, Games, Purchases, and current-year Statistics
• show inherited site defaults explicitly and keep source labels, tooltips, accessibility text, colors, and resolved values synchronized after live saves
• fix duplicate add-form element IDs exposed by DEBUG validation
• add server and mobile/desktop browser coverage, configuration docs, and changelog notes

Verification

• direnv exec . make check
• 1,955 Python/browser tests passed
• 583 Vitest tests passed

Closes #385

[github-actions]

Staging deployment: https://timetracker-staging-issue-385-settings.fly.dev