refactor: two-section sidebar with agent detail panel, theme system with 9 built-in themes

sidebar layout:
- top section: workspace list with accent bar, git branch, state dots
- bottom section: agent detail panel sorted by urgency with braille spinners
- active workspace always highlighted (subtle bg in terminal mode, brighter in nav)
- palette-driven colors throughout (no hardcoded colors in ui.rs)

theme system:
- 9 built-in themes: catppuccin (default), tokyo-night, dracula, nord, gruvbox, one-dark, solarized, kanagawa, rose-pine
- config: theme.name selects base, theme.custom overrides individual tokens
- 15 color tokens: accent, surface0/1, overlay0/1, text, subtext0, mauve, green, yellow, red, blue, teal, peach
- legacy ui.accent still works for backward compatibility

infrastructure:
- Palette struct on AppState, ready for future theme system extensions
- git_branch() reads .git/HEAD for branch detection
- PaneDetail struct with urgency sorting for agent panel
- spinner_tick counter for braille animation (~8fps on 60fps render loop)

1 files changed, 14 insertions, 2 deletions

diff --git a/src/main.rs b/src/main.rs
index 2341e09b..9d5d1b16 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -73,13 +73,25 @@ fn init_logging() {
         .init();
 }
 
-const DEFAULT_CONFIG: &str = r#"# herdr configuration
+const DEFAULT_CONFIG: &str = r##"# herdr configuration
 # Place this file at ~/.config/herdr/config.toml
 
 # Show first-run notification setup on startup.
 # Missing also shows onboarding; set false after you've chosen.
 # onboarding = true
 
+[theme]
+# Built-in themes: catppuccin, tokyo-night, dracula, nord, gruvbox,
+#                  one-dark, solarized, kanagawa, rose-pine
+# name = "catppuccin"
+
+# Override individual color tokens on top of the base theme.
+# Accepts: hex (#rrggbb), named colors, or rgb(r,g,b)
+# [theme.custom]
+# accent = "#f5c2e7"
+# red = "#ff6188"
+# green = "#a6e3a1"
+
 [keys]
 # Prefix key to enter navigate mode (default: "ctrl+b")
 # Examples: "ctrl+b", "f12", "esc", "-"
@@ -126,7 +138,7 @@ const DEFAULT_CONFIG: &str = r#"# herdr configuration
 [advanced]
 # Allow launching herdr from inside a herdr-managed pane.
 # allow_nested = false
-"#;
+"##;
 
 fn should_block_nested(config: &config::Config) -> bool {
     should_block_nested_for_env(config, std::env::var(HERDR_ENV_VAR).ok().as_deref())