feat(settings): new settings nav, save bar and shared pieces for the redesign - #1525

🟢 Confidence 8/10 · likely safe to merge
The draft hook's submit/abort/commit protocol is only exercised by tests here; its first production caller lands in #1526.
_{quality 100/100 · no findings · tests covered · risk medium}

Adds the settings-redesign foundation: a regrouped nav with redirects for every retired ?tab= id, an unsaved-changes guard and save bar backed by useSettingsDraft, and an Email preferences tab on /account. Every tab still renders its current content, so it merges on its own.

• useSettingsDraft tracks an in-flight save so a reverted edit stays dirty until the refetch lands
• SettingsSaveBar guards navigation on dirty with a confirm dialog and beforeunload prompt
• Settings nav regrouped into Workspace, Access, Data, Alerting with new tab ids
• Retired ?tab= ids redirect to their new tab, /integrations or /account

_{4950e5d · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}