feat: agent avatars, task tags, search, edit/delete agents, cron readability, timezone fix

- Agent avatars: first-letter + deterministic color avatar in all views
- Agent management: edit (name/description) and delete with confirmation
- Execution API: validates agent existence, returns 410 if deleted
- Task tags: JSON array field, editable with tag pills UI
- Task search: by name (?q=) and by tags (?tags=) and by status
- Task edit: simplified to name/tags/description only (not agent params)
- Cron display: human-readable Chinese (e.g. '每5分钟', '每天 09:00')
- Timezone fix: UTC→local via dayjs.utc().local() across all pages
- Content area: full width (removed max-width:1200px constraints)
- Login page: standalone layout without sidebar

backend/app/schemas/task.py

@@ -10,6 +10,7 @@ class TaskCreate(BaseModel):
     description: str = Field(default="")
     cron_expression: str = Field(..., max_length=64)
     grace_period: int = Field(default=300, ge=0)
+    tags: list[str] = Field(default_factory=list)
 
 
 class TaskUpdate(BaseModel):
@@ -18,6 +19,7 @@ class TaskUpdate(BaseModel):
     cron_expression: str | None = None
     grace_period: int | None = None
     status: str | None = None  # active / paused / stopped
+    tags: list[str] | None = None
 
 
 class TaskOut(BaseModel):
@@ -29,6 +31,7 @@ class TaskOut(BaseModel):
     cron_expression: str
     grace_period: int
     status: str
+    tags: list[str] = Field(default_factory=list)
     last_run_at: datetime | None = None
     last_run_result: str | None = None
     next_run_at: datetime | None = None