Add copy buttons to the API key displays

Reading the API key out of the settings panel meant selecting it by hand.
The pre blocks are styled user-select: all, so a single click already
selects the whole value, but nothing about them advertises that.

Adds renderCopyableValue(), which pairs a pre block with an
ExtraButtonComponent carrying the "copy" icon, and copyToClipboard(),
which writes via navigator.clipboard and confirms with a Notice. The
rejection path gets its own Notice rather than being swallowed: the
notice is the button's only feedback, so a silent failure would be
indistinguishable from success.

Both the REST and MCP sections now show two forms of the key: the
existing authorization header value, and the key on its own for tools
that ask for it without the Bearer prefix. Each has its own button that
copies exactly what is displayed, rather than one button copying
something other than what the user is looking at. The second block is
introduced with new prose so the two are not mistaken for duplicates.

navigator.clipboard is permitted by obsidianmd/platform, which bans only
navigator.userAgent and navigator.platform for OS detection; the plugin's
own test suite asserts navigator.clipboard.writeText as valid. Verified
by linting both calls.

The API key control on the Advanced settings page does not get a button:
it is a declarative text control, and SettingControlBase exposes no class
or render hook to attach one to.

No test coverage: src/main.ts has none today, and mocks/obsidian.ts lacks
Setting, Notice, and the createEl/createDiv HTMLElement augmentations
this code depends on. Covering it means building out that mock, which is
a larger job than this change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>