feat: switch between named GitHub profiles from the TUI

[sashml]

Profile switcher: switch GitHub contexts from inside the TUI

Adds named profiles so you can juggle multiple contexts — work vs. personal, public github.com vs. a GitHub Enterprise host — and switch between them from the TUI with a keypress, no restart or config editing.

Config ([[profiles]], back-compatible)

Each profile is a named body of the existing github/dashboard/cache/ui fields, plus an optional top-level active_profile. If no profiles are defined, the top-level config is treated as a single default profile — existing configs work unchanged.

active_profile = "work"

[[profiles]]
name = "work"
[profiles.github]
orgs = ["my-org"]
api_url = "https://api.github.com/graphql"
token_env = "GHDASH_TOKEN_WORK"   # env var NAME — never the token itself

[[profiles]]
name = "acme-enterprise"
[profiles.github]
orgs = ["acme"]
api_url = "https://ghe.acme.corp/api/v3"
token_env = "GHDASH_TOKEN_ACME"

Behavior

• p opens a centered, type-to-filter modal picker (arrows navigate, Enter switches, Esc cancels). The active profile is marked and always shown as a status-bar chip.
• Switching drops the old client, rebuilds it for the new profile's token and api_url, clears the PR set, and re-fetches.
• Per-profile token resolution: token_env → GITHUB_TOKEN → gh auth token --hostname <host> → GH_TOKEN. api.github.com maps to github.com for gh; Enterprise hosts are used as-is.
• Per-profile cache namespace (<cache-dir>/<profile-name>/) so switching never shows another profile's cached data.
• Secret hygiene: tokens are only ever held in memory and only sourced from env/gh — never written to config, cache, logs, or the UI.

Tests

Adds coverage for the config schema and back-compat, host mapping, token resolution order, cache isolation, the picker reducer, and a check that no token value appears in serialized config or rendered output. cargo test, cargo clippy --all-targets -- -D warnings, and cargo fmt --check all pass.