TUI: add config option to hide message source labels

[bug-ops]

Problem

Every chat message displays a source label prefix ([user], [zeph], [bash], [mcp], etc.). These labels add visual noise and are often redundant since message styling (color) already distinguishes roles. There is no way to disable them.

Proposed solution

Add a [tui] config section with show_source_labels = false (default).

Changes

1. crates/zeph-core/src/config/types.rs — add TuiConfig struct:

   pub struct TuiConfig {
       pub show_source_labels: bool, // default: false
   }

   Add pub tui: TuiConfig field to root Config.

2. config/default.toml — add section:

   [tui]
   show_source_labels = false

3. crates/zeph-tui/src/widgets/chat.rs — conditionally render labels:

   • Lines ~116-121: skip [user]/[zeph]/[system] prefix when disabled
   • Lines ~224-225: skip [bash]/[mcp] tool name prefix when disabled
   • Adjust indent calculation accordingly

4. Thread config to chat widget — pass show_source_labels bool through the existing render path (the App struct already has access to config).

Notes

• Default is false (labels hidden) since message colors already indicate the source
• Env override: ZEPH_TUI__SHOW_SOURCE_LABELS=true