feat(chat): add session context budget API and chat budget visibility panel

[Congregalis]

Summary

This PR introduces session-level context budget visibility in two parts:

• Backend: adds a lightweight session context budget API to estimate current context load.
• Frontend: adds a chat-side context budget indicator + hover details panel in Agent Detail chat.

Commits included:

• abf3322
• 601d5a2

What Changed

1) Backend: Session Context Budget Endpoint

Added:

• GET /api/agents/{agent_id}/sessions/{session_id}/context-budget

Returns minimal budget fields:

• window_size_messages
• session_messages_total
• estimated_tokens_current_window
• budget_tokens
• usage_ratio

Implementation notes:

• Estimates tokens from window message content using existing char-based estimator.
• Uses a fixed default budget_tokens baseline for now.
• Read-only endpoint; does not change chat behavior.

2) Frontend: Context Budget Indicator in Chat UI

Added session budget visualization in Agent Detail chat:

• Circular budget progress indicator near send controls.
• Hover card showing detailed context budget fields.
• Progress ring and percentages bound to usage_ratio.
• i18n labels for EN/ZH.

Notes / Scope

• This PR is observability only for context budget.

ScreenShots

Follow-up

To keep this PR focused and easier to review, I’ve kept it to the context‑budget endpoint and UI only, and would implement auto compaction logic separately if you think it’s a good fit for the project.

A follow‑up PR with auto compaction would:

• proactively reduce the risk of context overflow and hard failures when long‑running sessions grow large;

• avoid silent quality degradation from models receiving overly long, noisy histories;

Checklist

• Tested locally
• No unrelated changes included