fix(site): use search params for model edit/add navigation

[DanielleMaywood]

🤖 This PR was written by Coder Agent on behalf of Danielle Maywood.

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Problem

The models, providers, and usage sections under /agents/settings used local useState to toggle between list and detail/form views. Because no URL change occurred, the browser back button skipped over forms entirely and returned the user to whatever page they were on before the settings page.

Solution

Replace local view state with URL search params in all three sections:

Section	Search params
Models	?model=<id> (edit), ?newModel=<provider> (add)
Providers	?provider=<name> (detail)
Usage	?user=<username> (user detail)

Clicking a row now pushes a history entry so the browser back button returns to the list.

History management per action:

Action	Mechanism	Why
Click row	push	New history entry for the form
Back / Cancel	push (clear params)	Back returns to form you just left
Update	push (clear params)	Back returns to form with saved state
Create	navigate(-1)	Blank add form is stale after creation
Delete	navigate(-1)	Resource no longer exists

Changes

• api.ts: Added getUser(userIdOrName) method wrapping GET /api/v2/users/{user}.
• queries/users.ts: Added userByNameKey and userByName(username) query.
• ModelsSection.tsx: Swapped useState<ModelView> for useSearchParams with useMemo-derived view. Added clearModelView() helper that removes only model-related params.
• ProvidersSection.tsx: Same pattern — swapped useState<ProviderView> for useSearchParams with useMemo-derived view. Provider disappearance is now handled naturally by the derivation falling back to list mode.
• SettingsPageContent.tsx: Swapped useState<ChatCostUserRollup | null> for useSearchParams. Uses the new userByName query to fetch the selected user's profile for the detail header, independent of pagination state.
• SettingsPageContent.stories.tsx: Added 4 new stories for the usage tab: user list, empty state, user drill-in, and drill-in-and-back navigation.

[ibetitsmike]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

coder/site/src/pages/AgentsPage/SettingsPageContent.tsx

Line 187 in 1de324d

if (selectedUser) {

Drive usage drill-in from selected username, not profile data

The detail panel is now rendered only when selectedUserQuery returns data, so any getUser failure (temporary 5xx, auth mismatch, etc.) silently drops back to the list even though ?user= is set and the summary query may still be valid. This makes row clicks appear broken and removes any visible error/retry path for the profile fetch; the detail mode should be keyed off selectedUsername, with profile errors handled inside that mode.

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