Fix #36: replace HelpText with user-controllable announcements and visible text

[kellylford]

Closes #36 (partially — see issue comment for the remaining screen reader problem and options).

What this PR does

Replaces all AutomationProperties.HelpText entries that were invisible to sighted users with visible text, and routes the one long instructional string (address field tutorial) through the user-controllable AccessibilityHelper.Announce system instead.

• ComposeWindow: removes instructional HelpText from the address token field; delivers it as a first-focus Hint announce (_addressHintGiven flag, respects AnnounceHints setting). Removes redundant HelpText from From combobox.
• SettingsDialog: F7/Shift+F7 spelling note is now a visible TextBlock; spelling verbosity radio buttons now have visible TextBlock examples; redundant HelpText removed from Log Format and Message Open Mode radio buttons (those already had visible descriptions).
• NewFolderDialog: redundant HelpText removed.
• RulesManagerWindow: From/Body HelpText converted to visible TextBlock notes; Body AutomationProperties.Name updated to "Body preview contains"; HelpText removed from To and Subject fields.
• AccountManagerDialog / AddAccountDialog: visible "Stored in Windows Credential Manager." note added below password field; signature label updated to include "replies and forwards".
• CommandPaletteWindow: unchanged — gesture text in HelpText is intentional keyboard shortcut metadata.

Known limitation

The visible TextBlock elements added by this PR are not in the tab order and are not automatically read by screen readers. A JAWS user needs the JAWS cursor; an NVDA user needs object navigation to reach them. This is a WPF platform limitation: AutomationProperties.DescribedBy does not exist, GetDescribedByCore() is absent from AutomationPeer in all .NET versions through .NET 10, and PasswordBox is sealed. See the issue comment for the two options being considered for a follow-up pass.

Test plan

• Open Compose window — address hint announces once on first focus of any address field, suppressed on return visits
• Toggle AnnounceHints off in Settings — confirm address hint is suppressed
• Settings → Screen Readers tab: F7/Shift+F7 note visible below "Announce spelling while navigating" checkbox
• Settings → Screen Readers tab → Spelling Suggestions Verbosity: example lines visible below each radio button
• Settings → Advanced → Log Format: renders correctly, no regression
• Settings → Windowing → Message Open Mode: renders correctly, no regression
• New Folder dialog: works normally
• Rules Manager: From field shows domain-matching note; Body field shows preview note
• Account Manager / Add Account: "Stored in Windows Credential Manager." visible below password field; signature label reads "replies and forwards"

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