docs(ui-kit/ios,sdk/ios): pin & save messages and thread subscription

[shreeyajoshi-cometchat]

Description

Docs: Pin & Save Messages and Thread Subscription for iOS

Documents the two iOS v5 features that shipped in 5.1.19 with no documentation — Pin & Save Messages (ENG-37851..37853) and Thread Subscription (ENG-37571..37575).

New pages

• sdk/ios/pin-save-message — pin/unpin, save/unsave, list fetch via MessagesRequest + set(pinned:)/set(saved:), and the real-time delegate callbacks.
• sdk/ios/thread-subscription — subscribeToThread/unsubscribeFromThread, the tri-state threadSubscriptionState, ThreadsRequest and the MessageThread model, and the two CometChatThreadDelegate callbacks.
• ui-kit/ios/pinned-messages — CometChatPinnedMessages (per-conversation, opened from the message header).
• ui-kit/ios/saved-messages — CometChatSavedMessages (personal, cross-conversation).
• ui-kit/ios/guide-pin-save-message — end-to-end walkthrough.
• ui-kit/ios/guide-thread-subscription — the gate, the action-sheet option, the message-header bell, and cross-surface sync.

Updated pages

• message-list — enablePinMessage, enableSaveMessage, enableThreadSubscription, and the matching hide*Option flags.
• message-header — parentMessage, enableThreadSubscription, hideThreadSubscriptionButton (the subscription bell).
• events — ccMessagePinned / ccMessageSaved, plus a Thread Events section for ccThreadSubscriptionChanged.
• message-template — the new option ids.
• core-features, components-overview, guide-overview, guide-threaded-messages, sdk/ios/threaded-messages — feature entries and cross-links.
• docs.json — registered the new pages.

Two SDK pages used "pinned" as a tags example, which now reads as the pin feature; changed to "important" with a pointer to the real API.

Notes for review

• Thread-subscription copy is "Subscribe to thread" / "Unsubscribe from thread", carried by the THREAD_SUBSCRIBE / THREAD_UNSUBSCRIBE keys.
• The subscription option is offered on parent messages only, never on a reply, and is not gated on reply count. This differs from Android, which shows the option on replies too and re-roots the subscription to the thread's parent — worth a product call on whether iOS should match.
• Unlike React, iOS has no receiver-type check, so the option appears in one-on-one conversations as well as groups. The pages describe iOS's actual behaviour rather than calling the feature group-only — Android and Flutter also allow 1:1.

Related Issue(s)

Type of Change

• Documentation correction/update
• New documentation
• Improvement to existing documentation
• Typo fix
• Other (please specify)

Checklist

• I have read the CONTRIBUTING document
• My branch name follows the naming convention
• My changes follow the documentation style guide
• I have checked for spelling and grammar errors
• All links in my changes are valid and working
• My changes are accurately described in this pull request

Additional Information

Depends on cometchat-team/uikit-ios#935 — [iOS] Thread Subscription — finalised copy + header subscribe control (ENG-37575), which carries the renamed THREAD_* string keys and the CometChatMessageHeader subscription bell that these pages document. This PR should merge after #935 lands.

Screenshots (if applicable)

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