refactor(agent): keep the assistant in the nav, FAB and underlag flow only (#1557)

The founder wants the scattered per-page assistant buttons gone: the
assistant is reachable from the nav and the floating tab everywhere, so
in-page duplicates were noise. Removed the AgentSparkleButton call sites
(year-end, verifikat detail, supplier invoice detail, invoice editor)
and the now-orphaned component, the soft hand-off link in the Ny
verifikat modal (plus its i18n keys), and the transaction-row overflow
item. Kept: nav entry, floating tab, the Dokumentinkorg flow, and the
sanctioned "Skapa med assistent" split-button mode on /bookkeeping
(design.md convention 14).

The command palette's hand-off entries hardcoded the agent name "Anna";
they now use the identity from AgentSheetProvider like every other
affordance, and hide until agent onboarding is done (same gate as the
FAB).

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

messages/en.json

@@ -5412,7 +5412,6 @@
     "tab_accounts": "Chart of accounts",
     "new_entry_dialog_title": "New journal entry",
     "create_with_assistant": "Create with assistant",
-    "ask_assistant_handoff": "Let the assistant fill it in?",
     "loading_source_voucher": "Loading source voucher...",
     "copy_failed_title": "Could not copy journal entry",
     "copy_source_missing": "Source voucher not found.",

messages/sv.json

@@ -5412,7 +5412,6 @@
     "tab_accounts": "Kontoplan",
     "new_entry_dialog_title": "Ny verifikation",
     "create_with_assistant": "Skapa med assistent",
-    "ask_assistant_handoff": "Hellre låta assistenten fylla i?",
     "loading_source_voucher": "Laddar källverifikat...",
     "copy_failed_title": "Kunde inte kopiera verifikat",
     "copy_source_missing": "Källverifikatet hittades inte.",