feat(ai-chat): header badge shows current/max context, max from AI settings (#189)

The floating chat window's header badge flipped meaning — a live per-turn token
counter while streaming, the persisted context size at rest — so it "reset to 1"
on each prompt and conflated two different numbers. Replace it with a stable
"current / max" context badge (e.g. `572 / 200k`). The live "Thinking · N tokens"
inside the chat body stays; only the duplicate live counter is removed from the
header.

Max comes from a new admin setting "Context window (tokens)". The server resolves
it and attaches `maxContextTokens` to the completed assistant turn's metadata
(next to contextTokens), so the badge needs no client-side model resolution and
this survives public shares / per-role models.

Server:
- ai.types: chatContextWindow on AiProviderSettings + PROVIDER_SETTINGS_KEYS +
  ResolvedAiConfig + MaskedAiSettings.
- workspace.repo: chatContextWindow in AI_PROVIDER_SETTINGS_ALLOWED (parity).
- update-ai-settings.dto: @IsInt @Min(0) chatContextWindow.
- ai-settings.service: coerce the ::text-stored value to a positive int in
  resolve()/getMasked().
- ai-chat.service: flushAssistant writes metadata.maxContextTokens (>0); the
  completed turn passes resolved.chatContextWindow.

Client:
- ai-chat.types: maxContextTokens on the message-row metadata.
- ai-chat-window: read maxContextTokens; render "current [/ max]"; drop the
  liveTurnTokens state/branch and the onLiveTurnTokens prop; new tooltip.
- chat-thread: remove the live-turn-token throttle effect and plumbing.
- count-stream-tokens: drop the now-dead liveTurnTokens()/types; keep
  estimateTokens.
- settings: chatContextWindow on IAiSettings(+Update) + a NumberInput in the AI
  provider settings form.

i18n: add the badge/settings keys (en, ru); remove the two now-unused keys.
Tests: flushAssistant maxContextTokens, DTO validation, trim token tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/workspace/services/ai-settings-service.ts

@@ -22,6 +22,8 @@ export type ChatApiStyle = "openai-compatible" | "openai";
 export interface IAiSettings {
   driver?: AiDriver;
   chatModel?: string;
+  // Max context window in tokens shown in the chat header badge; 0/unset = no limit.
+  chatContextWindow?: number;
   chatApiStyle?: ChatApiStyle;
   // Cheap model id for the anonymous public-share assistant; empty = chatModel.
   publicShareChatModel?: string;
@@ -56,6 +58,8 @@ export interface IAiSettings {
 export interface IAiSettingsUpdate {
   driver?: AiDriver;
   chatModel?: string;
+  // Max context window in tokens for the chat header badge; 0 = clear the limit.
+  chatContextWindow?: number;
   chatApiStyle?: ChatApiStyle;
   publicShareChatModel?: string;
   // Agent-role id whose persona the public-share assistant adopts; empty =