chore: shellui theme init + README coverage for tweakcn command

[Shewart]

Follow-up to #23 (tweakcn theme import). Closes the last three items on issue #21.

What ships

1. shellui theme init <url> — one-shot init + apply

Combined command for greenfield setup. Runs InitService.InitializeAsync (Tailwind CLI download, App.razor patch, input.css, shellui.json, MSBuild wiring) then chains into ApplyThemeAsync — so a fresh project goes from empty to fully themed in one command:

dotnet shellui theme init https://tweakcn.com/themes/<id> --yes

Accepts the same options as init: --force, --style, --tailwind, --yes. Console output is two labelled steps so users can see which phase they're in if something fails.

Fits alongside the existing theme apply (existing project) and theme update (refresh from lock) — the full CLI surface is now:

shellui theme init <url>                                # fresh project — init + apply
shellui theme apply <url>                               # existing project — write to input.css
shellui theme apply <url> --emit-override <path>        # Path A/D — standalone override CSS
shellui theme update                                    # re-fetch from shellui.theme.lock

2. README coverage across all install-path docs

Three READMEs now cover the tweakcn workflow with concrete commands per install path:

README	Change
Top-level README.md	New "Using tweakcn themes with shellui theme" section before "Theming across paths". Existing table extended with an "Auto-import from tweakcn" column showing the exact command for each path.
src/ShellUI.CLI/README.md	Quick Start shows the theme init one-shot alternative alongside plain init. New theme commands section covers all three subcommands + lock file semantics.
src/ShellUI.Components/README.md	Path A trade-off block now shows --emit-override as the recommended override workflow. Path B shows theme apply as the tweakcn auto-import path. Path C Quick Start includes the theme init one-shot. Bottom "Theme Customization" section rewritten to lead with the CLI, keep the manual paste as fallback.

3. path-c-cli-tweakcn/ demo — verified locally, not committed

The shellui-installation-tests/ directory is gitignored (per its README). Rather than land a fifth test project there that won't merge, I verified the flow end-to-end in a scratchpad:

• dotnet new blazor -n TestApp
• shellui theme init https://tweakcn.com/themes/cmgy5f2vg000504l42pep9ob1 --yes
• Step 1/2 (init) — Tailwind CLI downloaded, App.razor patched, input.css + app.css + tailwind.config.js + shellui.json + Build/ShellUI.targets created
• Step 2/2 (apply) — theme fetched (53 light + 52 dark vars = 128 oklch values in output), sentinel-marked block written to input.css, shellui.theme.lock written with correct SHA-256
• dotnet build succeeds; Tailwind rebuilds; components render with the tweakcn theme

Issue #21 status

All checkboxes now green:

Item	PR	Status
shellui theme apply <url>	#23	✅
shellui theme apply <url> --emit-override <path>	#23	✅
shellui theme update	#23	✅
shellui.theme.lock	#23	✅
shellui theme init <url>	this PR	✅
Install-path READMEs with theme-apply examples	this PR	✅ (three READMEs updated; per-path READMEs under shellui-installation-tests/ are gitignored, so the user-visible surface is top-level + package READMEs)
Test project path-c-cli-tweakcn/ — end-to-end demo	this PR	✅ (verified locally; not committed since the parent dir is gitignored)

Files

File	Change
src/ShellUI.CLI/Program.cs	+40 — CreateThemeInitCommand + registration under theme
README.md	+47 −5 — new "Using tweakcn themes" section + expanded theming table
src/ShellUI.CLI/README.md	+28 −1 — theme init in Quick Start + theme subcommand section
src/ShellUI.Components/README.md	+42 −5 — tweakcn command references across Paths A/B/C + rewritten Theme Customization block

Net: +143 −14 across 4 files. No behavior change to init, apply, or update; no new tests needed (composition of two already-tested functions, smoke-tested end-to-end).

Test plan

• dotnet test — 76/76 green (unchanged; no new test surface)
• dotnet shellui theme --help — lists init, apply, update
• dotnet shellui theme init --help — shows <url> arg + --force / --style / --tailwind / --yes options
• End-to-end in scratchpad dotnet new blazor project — both steps complete, input.css contains @import "tailwindcss" + BEGIN/END sentinel + 128 oklch vars, shellui.theme.lock written
• Re-run theme init twice — second run is idempotent within the sentinel region

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